Issue 1453: MDS: fall back to SVD when the full similarity matrix is known

The SVD method can now be used to calculate the metric MDS. In order to do this a new argument ``method`` which defaults to ``smacof`` and can be set to ``svd`` was introduced in the MDS estimator.
This was done in order to fix issue #1453.
Should `X_smacof` and `X_svd` be approximately be equal up to a sign flip? If so you could check the equality of the component wise absolute values.